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What's the difference between 2-stroke and 4-stroke engines?

Fuel for a 2-stroke engine has a small amount of oil mixed into it. It is called a "2-stroke" because just one up and down movement of the piston—the 2 strokes—performs the full cycle of intake, compression, combustion and exhaust. No intake or exhaust valves are used and instead, small holes called scavenging ports in the cylinder wall are used for drawing in air and expelling exhaust. Because combustion takes place with each revolution of the crankshaft with a 2-stroke, this format puts out more power than a 4-stroke engine and the power has more instantaneous delivery. This are some reasons why 2-stroke engines have a long history of use on many different types of motorcycles.
However, concern for more environmentally friendly performance grew and 4-stroke engines are now the norm because they inherently have better fuel economy and less exhaust smoke. As of 2019, the only 2-stroke motorcycles Yamaha makes are for closed-course competition and some models for export. Nonetheless, Yamaha 2-stroke products have a simple, lightweight construction and comparatively easy maintenance, and their high reliability keeps them popular in many regions. Today, Yamaha 2-stroke snowmobiles are used to traverse the icy cold environment of Russia, and our 2-stroke outboard engines are widely used across Africa for fishery. And, many motorcycle enthusiasts continue to love 2-strokes for their punchy, breathtaking feeling of acceleration.
As for 4-stroke engines, they run on gasoline without any oil mixed in and the piston goes up and down two times for every combustion cycle, hence it's called a "4-stroke." However, 4-stroke engines require valves for both the intake and exhaust that must operate with high precision, making this engine format more complex, heavier and other disadvantages. But they provide stable power delivery, good fuel efficiency, cleaner emissions and more. This is why almost all two-wheelers, from big motorcycles to small scooters, use 4-stroke engines.